Particle image velocimetry measurements of transition flows in an 84-pin hexagonal rod bundle with spacer grids

Gas Fast Reactors (GFR) are a new type of nuclear reactor classified as a generation IV reactor. GFRs utilize a rod bundle arrangement in a triangular lattice due to the increased packing fraction and power density. Staggered spacer grids are commonly utilized to maintain the lattice geometry and to reduce flow-induced vibrations. Spacer grids generate a region of flow disturbance which requires comprehensive experimental investigation. Currently, there is a lack of high-fidelity experimental data related to the flow characteristics of a spacer grid in the transitional regime. The presented investigation addresses the lack of experimental flow parameters by investigating the effects of a 3D printed staggered spacer grid in a triangular lattice fuel bundle using Matched Index of Refraction (MIR) and Time-Resolved Particle Image Velocimetry (TR-PIV) at a transitional Reynolds number of 3000. Three distinct planes of interest were studied: a nominal plane, a near-center plane, and a center plane. The flow effects are analyzed by identifying phenomena present in two-dimensional two-component TRPIV first- and second-order parameters such as velocity magnitude, mean velocities, root mean square velocity fluctuations, Turbulent Kinetic Energy (TKE), Reynolds shear stress, and vorticity. Line-profiles are presented to identify the downstream effects of the TR-PIV parameters as well as providing quantitative analysis for numerical validation. Two-point spatial cross-correlation was incorporated in this study to identify the spatial effects caused by the spacer grid. Finally, Power Spectral Density (PSD) analysis was performed to quantify the frequency distribution of spanwise and streamwise velocity fluctuation, providing insights into the dominant spatial frequencies and turbulent patterns present in the data. This experimental campaign provides flow dynamics insights and high-fidelity datasets for the complex flow past spacer grids in a prototypical GFR fuel assembly which is valuable toward the design and safety development of GFRs.